A continuation of structural investigations on expressed domains from Complement Receptor CR2 is proposed. Complement receptor type 2 (CR2, CD21) normally serves as a receptor for complement C3 activation fragments and the immuno-modulatory protein CD23. Expression by B lymphocytes is required for the development of a normal humoral immune response to foreign antigens. This is due to the ability of CR2 to functionally link the binding of C3 fragments iC3b and C3d,g with antigen to the signal transducing capability of the B lymphocyte protein CD19. In addition the Epstein-Bar virus (EBV) utilizes CR2 as its primary means of infecting cells. The investigators along with other groups have previously studied structure-function relationships that govern the interaction of CR2 with the above ligands using strategies involving mutagenesis, and antibody binding. They have found that a domain composed of two 60-70 amino acid segments called N-terminal short consensus repeats (SCRs) contain the only two binding sites for C3 fragments and the EBV protein gp350/220, and contain one of the two sites for CD23. A model for this domain of CR2 and the location of the CD3 binding sites has been created, but the actual three dimensional structure is not known. The investigators propose to apply an NMR method for determining the three dimensional structure of the CR2 domain. They propose to further characterize ligand binding sites using mutagenesis and inhibitor techniques. They also propose to determine how this domain interacts with other regions of CR2. More specifically, their three specific aims are: 1)to determine the structure at high resolution of the human CR2 SCR 1-2 domain. 2)to define the ligand binding sites in the human CR2 SCR 1-2 domain for C3 activation fragments iC3b and C3d,g in addition to binding sites for EBV gp350/220 and CD23. 3) to establish the 3-dimensional solution phase structure of a longer CR2 region containing the SCR 1-2 domain and determine whether the orientation of this domain is influenced by its carboxy-terminal flanking SCRs.